I always had poor posture.
I have a light form of scoliosis, other people in my family have it also but they seem to stand straight while my shoulders are rounded and my stomach sticks out.

When i do only pull ups and swim it seems to be better but when i skip rope or run for ex it gets worse.

The strange thing i noticed, when i do crunches or sit ups and especially leg raises my posture gets really bad,
it looks like i have a huge stomach/ belly sticking out, a cavzman look you might say…
Does somebody experiences this also and know any corrective exercises? i want to build my abs but cant do them due to the effect on my posture

So yeah. Core exercises for you. You don’t have to stop doing crunch type things, just do the core stuff as well, although if it’s really bad you might want to drop crunch movements for the time being and get the rest of your core up to snuff first.